Imaging systems for ionising radiation

ABSTRACT

Flat panel images obtained during concurrent radiotherapy typically suffer from artefacts that relate to the pulses of MV energy. For a radiotherapeutic apparatus comprising a pulsed source of therapeutic radiation, a detector comprising control circuitry, an array of pixel elements, each having a signal output and an ‘enable’ input and being arranged to release a signal via the signal output upon being triggered by the enable input, and an interpreter arranged to receive the signal outputs of the pixel elements, the interpreter having a reset control, there are advantages in the control circuitry being adapted to reset the interpreter after a pulse of therapeutic radiation, prior to enabling at least one pixel of the array. Alternatively, the control circuitry can prompt a plurality of pulses by the pulsed source and then enable a plurality of pixels of the array. In effect, the therapeutic pulses are grouped into a short flurry of pulses. It is therefore preferred that the plurality of pixels comprises substantially all the pixels of the array.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Section 371 National Stage Application ofInternational Application No. PCT/EP2006/008905, filed Sep. 13, 2006 andpublished as WO 2008/031443 A1 on Mar. 20, 2008, the content of which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to imaging systems for detecting ionisingradiation.

BACKGROUND ART

Flat panel images are often used in radiotherapy and other applicationsin order to derive an image from ionising radiation that has (forexample) passed through a patient. Diagnostic images are generallyobtained from the use of kilovoltage (kV) radiation and can be usedeither as two dimensional images or can be used as one of a number ofsuch two dimensional images in order to create three dimensionalrepresentations via computed tomography (CT). Therapeutic radiationtends to be in the megavoltage (MV) range and can also be used to derivea portal image. This is an image of the therapeutic radiation after ithas passed through the patient; generally the image is of a low qualitywith poor contrast. Anatomical features are however apparent in theimage and can be used to check (for example) that the patient iscorrectly positioned.

MV radiation produces an image with inherently low contrast andtherefore it is important that no artefacts are present in the image toobscure the anatomy. The MV source typically operates in a pulsed mannerat a duty cycle of approximately 1 in 1000 and therefore there is ampleopportunity between pulses to obtain data from a few rows of the flatpanel imager.

FIG. 1 shows the typical structure of a flat panel imager 10. An upperlayer 12 consists of a scintillator under the application of x-rays 14.The light 16 thus produced impinges on an array 18 of photodiodes andtransistors which are disposed in a layer immediately beneath the upperlayer 12. The array 18 is divided into individual pixels, each of whichis associated with a single photodiode. The light impinges on thephotodiode in the array and creates an electronic signal which is gatedappropriately by the transistor. The electronic signal 20 thus producedis extracted from the flat panel array via read-out electronics 22 toform a digital data stream 24 that is used to construct the image.

SUMMARY OF THE INVENTION

We have found that such portal images suffer from artefacts that relateto the pulses of MV energy. When a MV pulse arrives, it will not onlycause scintillation in the scintillator, but will also impinge on thetransistor array and the read-out electronics and ionise the material ofwhich they are formed. This will therefore create a further electronicsignal, entirely bypassing the scintillator and the photodiodes. Wepropose two ways in which these problems can be overcome.

In a first aspect, therefore, we provide a radiotherapeutic apparatuscomprising a pulsed source of therapeutic radiation and a flat paneldetector, the detector comprising control circuitry, an array of pixelelements, each having a signal output and an ‘enable’ input and beingarranged to release a signal via the signal output upon being triggeredby the enable input, and an interpreter arranged to receive the signaloutputs of the pixel elements, the interpreter having a reset control,the control circuitry being adapted to reset the interpreter after apulse of therapeutic radiation, prior to enabling at least one pixel ofthe array.

Thus, the interpreter reset remains on during (or is activated after) atherapeutic pulse. This then causes the system to ignore the chargecollected as a result of the pulse and removes artefacts resultingtherefrom.

Generally, the pixel elements of such detectors work by outputting asignal in which the total charge passed reflects the total incidentradiation since the last time the pixel was read. As radiation isincident in the pixel, it causes ionisation and the resulting charge isretained. When the pixel is enabled, that charge flows via the outputand needs to be counted. Thus, the interpreter usually comprises anintegrator, in which case the reset control is arranged to zero theintegrator. Integrators are used to integrate the current into thephotodiodes and thereby measure the charge that has passed.

The second approach to the problem lies in a reassessment of one of theassumptions behind the operating mode of the therapeutic accelerator. Asmentioned above, this operates in a pulsed manner, with a duty cycle ofapproximately one in a thousand. Typically, this is a 3 μs pulse every 3ms or thereabouts.

The total time required for a treatment will ideally be minimised. Astime passes, the patient tires and may move or involuntary internalmotion may occur, meaning that long treatment times can be lessclinically effective. Further, a reduced treatment time can allow morepatients to be treated, thereby increasing the clinical efficiency ofthe apparatus. Thus, from the desire to deliver the treatment in theminimum possible time, it can be inferred that there must be a very goodreason indeed for the adoption of a duty cycle as low as 0.1%. Even aduty cycle as low as 1% would allow a tenfold improvement in clinicalefficiency together with an (unqualified) improvement in clinicaleffectiveness.

That reason is the protection of the apparatus from thermal overload.The pulses that are delivered contain a high energy, and the energyrequired to produce them is also large. This energy must be dissipated,and (inevitably) some will appear as heat in the apparatus. The dutycycle needs to be low in order to prevent the temperature of the devicebecoming unacceptably high. The chosen duty cycle is that at which therate of heat input to the device matches the rate of cooling that isprovided.

We have realised that the existence of a time constant in the thermalbehaviour of the device means that, in fact, it is the average dutycycle over the time constant that is important in this balance. If theduty cycle temporarily increases and then decreases, the apparatus maystart to warm but will then cool before exceeding the limits ofacceptable temperatures.

In a second aspect of the invention, therefore, we therefore provide aradiotherapeutic apparatus comprising a pulsed source of therapeuticradiation, a detector, and control circuitry for the pulsed source andthe detector, the detector comprising an array of pixel elements, eachhaving a signal output and an ‘enable’ input and being arranged torelease a signal via the signal output upon being triggered by theenable input, the control circuitry being adapted prompt a plurality ofpulses by the pulsed source and then enable a plurality of pixels of thearray.

In effect, the therapeutic pulses are grouped into a “pulse of pulses”—ashort flurry of pulses. During this time, the rate at which theapparatus is heated will exceed the rate at which it is being cooled,and the temperature can be expected to rise. However, it can be followedby longer period of downtime during which the apparatus will cool.During that downtime, the image can be collected from the imaging panel.

It is naturally preferred that the plurality of pixels comprisessubstantially all the pixels of the array. However, it is possible tocollect part of the panel prior to delivering a further pulse of pulses.Generally, the arrays of pixels are two dimensional for ease ofanalysis. To speed the collection of images, the enable inputs of agroup of pixels are connected in common thereby to enable the wholegroup simultaneously. If the pixels are grouped in rows within the 2Darray, then that entire row can be read out at the same time. Theoutputs of each column of pixels can be passed to the interpreter orother output via a common output path.

Typical systems today have a maximum pulse rate of 400 to 600 pulses persecond. The use of the above-described invention can allow this pulserate to be increased to 1000 or even 1500 pulses per second during theperiods while pulses are being produced.

The apparatus can include a separate source of diagnostic radiation, orthe radiation for imaging purposes can be produced by a suitabletherapeutic source, either by way of a portal image, or by controllingthe source to produce lower energy radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying figures, in which;

FIG. 1 shows the schematic structure of a flat panel imager;

FIG. 2 shows a detailed image of a small group of pixels in the flatpanel imager;

FIG. 3 shows a typical timing scheme for the flat panel imager of FIG.2;

FIG. 4 shows a first revised timing scheme for such a flat panel imager;

FIG. 5 shows a typical timing scheme for a flat panel imager in thecontext of the arriving MV pulses;

FIG. 6 shows a further revised timing scheme; and

FIG. 7 shows the effect of the timing scheme of FIG. 6 on the thermalproperties of the accelerator.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIGS. 1 and 2 show an essentially standard imaging panel. FIG. 1 hasbeen described already and shows the vertical cross-section through asingle pixel. FIG. 2 shows sixteen pixels of the panel, a small portionof the entire panel but sufficient to explain the manner in whichmultiple pixels are read.

Referring to FIG. 2, therefore, the pixel array 26 is arranged in arectilinear manner with the pixels in straight rows and columns. Theintersection of a particular row 28 with a particular column 30therefore defines a specific pixel 32. Each pixel has an associatedtransistor 34 to gate its output, and each row has a common “enable”line 36 which activates the transistor 34 of every pixel in that row.

Each column has a common output line 38 which allows the charge that hasaccumulated on each pixel to escape to an integrator 40 where it ismultiplexed with the output of other columns. This allows the entire rowto be read at the same time.

Scanning control electronics 42 therefore enables each row sequentially,and the whole row is read at substantially the same time. The integratoris then reset, and the next row is enabled. This timing scheme is shownin FIG. 3. A “row enable” signal RE is sent to each row in turn. Afterrow ‘n’ has been enabled and the RE signal has finished, an “integratorreset” signal IR is sent to prepare the integrator for the signal fromrow ‘n+1’, and so on.

This does not however take account of the MV pulse that arrives fromtime to time. As noted above, that pulse causes ionisation in the rowenable lines 36, the output lines 38, the transistors 34, and theintegrator 40. All this adds to the charge collected by the integratorand will usually be sufficient to increase or perhaps saturate thesignal, leading to a distinctive white line across an entire row.

FIG. 4 shows the revised timing schedule according to an embodiment ofthe invention. This figure shows the Megavoltage pulse (MVP) which takesplace at a time dictated by the treatment control systems. The triggersignal for this pulse is also fed to the scanning control electronics,which prompts the previous IR signal to remain ‘high’ and for the REsignal to be suspended, until after the MVP has ceased. In effect, theIR signal is extended as shown at 44 until after the MVP ends, duringwhich time the RE signal is suspended. The IR signal then ends, and theRE signals continue their cycle.

In this way, the aberrant signal delivered to the integrator by the MVPis substantially allowed to dissipate and the integrator is left zeroedafter the MVP, ready to receive the signal from the next row. Mostpanels will in practice contain some parasite capacitance and resistancewhich prevents all the charge being removed from the panel, but thistechnique will substantially reduce the effect.

FIGS. 5 and 6 show the second embodiment of the invention. FIG. 5 showsthe MVP and RE traces for a known system but on a much longer timebasethan FIG. 3. Thus, several MVP triggers can be seen, along with severalRE triggers between them. In general, between each MVP trigger thesystem has been able to read several rows of the panel.

FIG. 6 shows how this timing scheme is modified according to the presentinvention. The MVP triggers are grouped into a short flurry 46, with noRE triggers between them. After that flurry, the RE triggers resume andare continued until (in this case) the entire array has been read. Afterthen, a new flurry of MVP triggers is delivered, and the panel isre-scanned by a fresh series of RE triggers.

This means that, during the flurry 46 of MVP triggers, the rate of heatinput into the device is temporarily higher. FIG. 7 shows a steady stateof heat input 48 for a known system, which is the time-averaged heatinput resulting from the MVP triggers of FIG. 5. This produces a steadytemperature 50 which is the temperature at which the rate of heat inputis balanced by the cooling systems provided. Adjacent the steady heatinput 48 is shown the heat input 52 (over time) that is produced by therevised timing schedule of FIG. 6, in which there are regular peaks 54during a flurry of MVP triggers 46, between which the rate of heat inputfalls to zero 56. This gives the temperature profile 58, a sawtooth witha rising edge 60 during the peaks 54 and a falling edge 58 between theflurries 46.

Clearly, if the higher rate of MVP triggers of a flurry 46 were to bemaintained indefinitely, the apparatus would overheat. However, acritical temperature condition takes time to come into existence as thethermal mass of the device must be raised to the higher temperature bythe application of sufficient heat energy. Therefore, there is time fora flurry 46; this moves the MVP triggers to create a gap during whichthe panel can be read. This means that either there are no MVP-inducedartefacts in the image, or that the artefacts are confined to the firstrow that is read. In the latter case, that row can be arranged to be atan edge and therefore ignored.

Clearly, both embodiments could be combined, with the first embodimentbeing employed to prevent any artefacts appearing on even the first rowof an image read according to the second embodiment. However, they arealso susceptible to independent application.

It will of course be understood that many variations may be made to theabove-described embodiment without departing from the scope of thepresent invention.

1. A radiotherapeutic apparatus comprising a pulsed source oftherapeutic radiation and a detector, the detector comprising controlcircuitry, an array of pixel elements, each pixel element having asignal output and an ‘enable’ input and being arranged to release asignal via the signal output upon being triggered by the enable input,and an interpreter arranged to receive the signal outputs of the pixelelements, the interpreter having a reset control; the control circuitrybeing adapted to reset the interpreter after a pulse of therapeuticradiation, prior to enabling at least one pixel element of the array. 2.The radiotherapeutic apparatus according to claim 1 in which the pixelelements output a signal in which the total charge passed reflects thetotal incident radiation since a predetermined instance.
 3. Theradiotherapeutic apparatus according to claim 2 in which the interpretercomprises an integrator.
 4. The radiotherapeutic apparatus according toclaim 3 in which the reset control is arranged to zero the integrator.5. A radiotherapeutic apparatus comprising a pulsed source oftherapeutic radiation, a detector and control circuitry for the pulsedsource and the detector, the detector comprising an array of pixelelements, each having a signal output and an ‘enable’ input and beingarranged to release a signal via the signal output upon being triggeredby the enable input, the control circuitry being adapted to prompt aplurality of pulses by the pulsed source and then enable a plurality ofpixel elements of the array.
 6. The radiotherapeutic apparatus accordingto claim 5 in which the plurality of pixel elements comprisessubstantially all the pixels of the array.
 7. The radiotherapeuticapparatus according to claim 5 in which the array is two dimensional. 8.The radiotherapeutic apparatus according to claim 7 in which the enableinputs of a group of pixel elements are connected in common thereby toenable the whole group simultaneously.
 9. The radiotherapeutic apparatusaccording to claim 8 in which the pixel elements are grouped in rowswithin the 2D array.
 10. The radiotherapeutic apparatus according toclaim 8 in which the signal outputs of a plurality of pixel elements,each pixel element being of a different group, are passed to theinterpreter via a common output path.
 11. The radiotherapeutic apparatusaccording to claim 5 in which the pixel elements output a signal inwhich the total charge passed reflects the total incident radiationsince a predetermined instance.
 12. The radiotherapeutic apparatusaccording to claim 5 in which the pulse rate of the source is variable.13. The radiotherapeutic apparatus according to claim 5 in which thepulse rate of the source is at least 1000 pulses per second.
 14. Theradiotherapeutic apparatus according to claim 5 in which the pulse rateof the source is at least 1500 pulses per second.
 15. Theradiotherapeutic apparatus according to claim 5 further comprising aseparate source of diagnostic radiation.
 16. (canceled)
 17. Theradiotherapeutic apparatus according to claim 6 in which the array istwo dimensional.
 18. The radiotherapeutic apparatus according to claim 5in which the enable inputs of a group of pixel elements are connected incommon thereby to enable the whole group simultaneously.
 19. Theradiotherapeutic apparatus according to claim 6 in which the enableinputs of a group of pixel elements are connected in common thereby toenable the whole group simultaneously.
 20. The radiotherapeuticapparatus according to claim 9 in which the signal outputs of aplurality of pixel elements, each being of a different group, are passedto the interpreter via a common output path.
 21. The radiotherapeuticapparatus according to claim 7 in which the pixel elements output asignal in which the total charge passed reflects the total incidentradiation since a predetermined instance.